Systems and methods for recovering an out-of-service node in a hierarchical network

ABSTRACT

Systems and methods for communicating with a radio frequency (RF) device are disclosed herein. In particular, a method of recovering an out-of-service node comprises sending a hailing message on a plurality of hailing channels from a second node to the out-of-service node, the second node being in direct wireless communication with the out-of-service node, and sending a synchronization request message from the second node to the out-of-service node on a first data channel.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/339,655, filed Dec. 29, 2011, and entitled “Systems and Methods forDynamic Squelching in Radio Frequency Devices,” which is acontinuation-in-part of U.S. patent application Ser. No. 13/283,526,filed Oct. 27, 2011, issued as U.S. Pat. No. 8,660,134, and entitled“Systems and Methods for Time-Based Hailing of Radio Frequency Devices,”each of which the entirety of is hereby incorporated herein by thisreference.

TECHNICAL FIELD

The present disclosure generally relates to the operation of radiofrequency devices, and more particularly relates to synchronizing datacommunications between radio frequency devices.

BACKGROUND

Typically, utility meters (e.g., gas meters, water meters, andelectricity meters) are read manually by meter readers who are employeesor contractors of the various utility providers. Manual meter readingrepresents a significant cost to a typical utility provider. With theadvent of wireless technology including mesh networking, utilityproviders have sought methods and systems for remote reading of watermeters and/or remote control of water supply valves.

Advanced Metering Infrastructure (AMI) or Advanced Metering Management(AMM) are systems that measure, collect and analyze utility data usingadvanced metering devices such as water meters, gas meters andelectricity meters. The advanced metering devices combine internal datameasurements with continuously available remote communications, enablingthe metering devices to transmit and receive data through the AMInetwork. In a typical configuration, an advanced metering device, suchas an advanced water meter, measures and collects usage data, such aswater usage data, at a customer's location. The metering device thenuses a built-in communication interface to transmit data to a parentnode up the network, often in response to the parent's request for suchinformation. In this way, utility providers may remotely “read” customerusage data for billing purposes.

SUMMARY

The present disclosure relates to systems and methods for communicatingwith a radio frequency (RF) device. An exemplary method includesrecovering an out-of-service node by sending a hailing message on aplurality of hailing channels from a second node to the out-of-servicenode, the second node being in direct wireless communication with theout-of-service node, and sending a synchronization request message fromthe second node to the out-of-service node on a first data channel.

Various implementations described in the present disclosure may includeadditional systems, methods, features, and advantages, which may notnecessarily be expressly disclosed herein but will be apparent to one ofordinary skill in the art upon examination of the following detaileddescription and accompanying drawings. It is intended that all suchsystems, methods, features, and advantages be included within thepresent disclosure and protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and components of the following figures are illustrated toemphasize the general principles of the present disclosure.Corresponding features and components throughout the figures may bedesignated by matching reference characters for the sake of consistencyand clarity.

FIG. 1 is a block diagram of the network topology of an AMI mesh networkin accord with one embodiment of the present disclosure.

FIG. 2 is a block diagram of a radio frequency circuit in accordancewith one embodiment of the present disclosure.

FIG. 3 is a flow diagram of the SLEEP state of an RF device in accordwith one embodiment of the present disclosure.

FIG. 4 is a flow diagram of the SLAVE state of the RF device of FIG. 3.

FIG. 5 is a flow diagram of the MASTER state of the RF device of FIG. 3.

FIG. 6 is a graph of power usage in an exemplary system in accord withone embodiment of the present disclosure.

FIG. 7 is a flow diagram showing a process for receiving a timesync inaccord with one embodiment of the present disclosure.

FIG. 8 is a diagram of hailing groups used a system hailing timer inaccord with one embodiment of the present disclosure.

FIG. 9 is a flow diagram of an exemplary embodiment of a hailing andretry process used by an RF device in MASTER state in accord with oneembodiment of the present disclosure.

FIG. 10 is a block diagram of a simple network infrastructure stemmingfrom a utility provider having a plurality of nodes in accord with oneembodiment of the present disclosure.

FIG. 11 is a flow diagram of a dynamic squelching method used in RFdevices in accord with one embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure describes systems and methods for operating radiofrequency (RF) devices such as Advanced Metering Infrastructure devices(AMI) using frequency-hopping spread spectrum technology. Moreparticularly, the systems and methods disclosed herein relate totime-based hailing of radio frequency devices.

Existing AMI deployments rely on and utilize mesh networks and meshnetworking devices to transmit and to receive data between nodes withinthe utility provider's network. Many of these devices employfrequency-hopping spread spectrum (FHSS) technology in compliance withFederal Communications Commission (FCC) rules and regulations part 15(47 C.F.R. §15). FHSS is a method of transmitting and receiving radiosignals by rapidly switching among many frequency channels using apseudorandom channel sequence known to both the transmitting andreceiving devices.

Because of the remote placement nature of the advanced metering devices,it is desired to maximize battery life of the metering devices in orderto reduce down time and to reduce the amount of maintenance that must beperformed on the metering devices. Similarly, it is desired to maximizeresponsiveness in communications between the advanced metering devicesand the utility provider network while complying with FCC rules andregulations. It is also desired to reduce interference and backgroundnoise that may cause communication failures and that may furtherdecrease battery life of the advanced meters.

While the present disclosure relates to mesh networking, as those havingordinary skill in the art will recognize, the present disclosure may beutilized in other types of networking environments, such aspoint-to-point FHSS networks as well.

As used herein, a MASTER state device, a device in MASTER state, or amaster device is a device that is attempting to send data to anotherdevice. A SLAVE state device, a device in SLAVE state, a slave device,or a target device is a device to which the master is attempting to senddata. As used herein, “parent” and “child” nodes should not be confusedwith “MASTER state” and “SLAVE state” devices. MASTER state and SLAVEstate are merely states/modes for each device. “Parent” and “child”nodes have a predefined relationship based on hierarchy (i.e., a parentnode such as device 110 in FIG. 1 is further up the hierarchy inrelation to the utility provider 102 than a child node such as devices112, 114, 116 in FIG. 1). Although the present disclosure describes asingle parent to multiple child relationship, it should be understoodthat multiple parents may exist within the same network. Further, achild may have multiple parents. In various embodiments, a single parentmay be paired with a single child. As an example, child nodes mayrepresent individual customers' utility meters while a parent node mayrepresent a primary data collection device responsible for collectingdate from and sending data to each child device. This configurationrequires that the parent device's system time be highly accurate.

Utility companies must periodically determine customer usage by takingmeter readings. To facilitate this process and to reduce costs to theutility companies, utility meters in the present disclosure may transmitusage data wirelessly through a network, such as a mesh network, back tothe utility provider. To reduce costs further while increasingreliability, it is an object of the present disclosure to increasebattery life while increasing data transmission reliability and reducingsystem response times. To do this, each of the utility company's AMI RFdevices is in one of 3 states: SLEEP state used to conserve batterylife; SLAVE state used for responding to and receiving data from aMASTER state device; and MASTER state used to initiate communicationswith (i.e., “hail”) and send data to a SLAVE state device.

In SLEEP state, a device partially awakens and briefly listens for a“hailing” signal on a hailing channel from another device in MASTERstate. If the device in SLEEP state fails to detect a hailing signal,the device remains in SLEEP state and periodically partially awakens tolisten for a hailing signal. The SLEEP state device changes hailingchannels based on a predefined pseudorandom hailing channel frequencyset dependent upon a system time. Once the SLEEP state device is“hailed” by a MASTER state device, it fully awakens and begins listeningfor data messages from the MASTER state device on a predefined datachannel selected from the predefined pseudorandom data channel frequencyset, the data channel being indicated by the MASTER state device. Inother words, the SLEEP state device exits SLEEP state and enters SLAVEstate.

In SLAVE state, a device listens for and receives data messages on adata channel selected from the predefined pseudorandom data channelfrequency set. The MASTER state device indicates which data channel touse by sending a data channel pointer to the target device during thehailing process. After receiving each message from the MASTER statedevice, the SLAVE state device sends an acknowledgement (ACK) message tothe MASTER state device, indicating a successfully received datamessage. The SLAVE state device and the MASTER state device then switchto the next data channel in the data channel frequency set and continuecommunications until all data messages have been sent.

In MASTER state, a device “hails” a SLEEP state device by sending ahailing signal on a hailing channel to the targeted SLEEP state device.The MASTER state device selects which hailing channel to use basedon: 1) the SLEEP state device's predefined pseudorandom hailing channelfrequency set, 2) a system time corresponding to the hailing channelfrequency set, and 3) a unique serial number (the “nodeID”) of the SLAVEstate device. The system time on the MASTER state device and the systemtime on the SLAVE state device are substantially in synchronization.Upon successfully “hailing” the sleeping device (which upon hailingbecomes a SLAVE state device), the MASTER state device begins sendingdata messages on a data channel to the SLAVE state device. The datachannel is selected from the SLAVE state device's predefinedpseudorandom data channel set based on the system time. In oneembodiment, the data channel frequency set is common to the MASTER statedevice and the SLAVE state device. In such a configuration, the MASTERstate device may indicate to the SLAVE state device during the hailingprocedure what the next available data channel is by sending to theSLAVE state device a data channel pointer.

Hailing channels and data channels are selected from the 902-928 MHzindustrial, scientific, and medical (ISM) bandwidth. In one embodiment,one hundred (100) channels are chosen with a minimum channel spacing of100 kHz each. Fifty (50) of the channels are randomly assigned to thepseudorandom data channel frequency set, and fifty (50) differentchannels are randomly assigned to the hailing channel frequency set. Theset of fifty (50) hailing channels are used during the MASTER and SLEEPstates to send and receive hailing requests while the set of fifty (50)data channels are used during the MASTER and SLAVE states to send andreceive data messages.

In one embodiment, a particular radio frequency device selects aninitial subset of two (2) consecutive channels (i.e., a channel group)from its predefined pseudorandom hailing channel frequency set to beused while in the SLEEP state (by first calculating a channel offsetbased on its unique assigned serial number (the “nodeID”)). This offsetis added to a hailing channel pointer. The hailing channel pointerpoints to one of the fifty (50) available hailing channels, andincrements to the next set of two (2) channels every, for example, 18seconds so that each device will continuously “hop” through all of thefifty (50) available hailing channels at a system hopping rate. In thismanner, hailing channel usage is spread across the predefined hailingchannel. In one embodiment, the hailing channel usage may besubstantially equal manner such that each channel within the hailingchannel frequency set is used for substantially the same amount of timeor for substantially the same number of times. In one embodiment, thehailing channel usage might be skewed to use hailing channels with lessinterference more frequently while using hailing channels with moreinterference less frequently. When sending and receiving data messagesin MASTER and SLAVE states, the devices hop through the data channelfrequency set to assure that, on average, all data channels are usedequally.

Pseudorandom Frequency Sets

As will be further understood, the present disclosure utilizes twopseudorandom frequency sets: a predefined pseudorandom hailing channelfrequency set and a predefined pseudorandom data channel frequency set.Hailing channels and data channels are randomly selected from the902-928 MHz ISM radio bandwidth. In one embodiment, one hundred (100)channels are chosen having a minimum channel spacing of 100 kHz each.Fifty (50) of the channels are randomly assigned to the hailing channelfrequency set, and fifty (50) different channels are randomly assignedto the data channel frequency set. In one embodiment, a different numberof total channels, a different number of hailing channels, and/or adifferent number of data channels may be used. In one embodiment, thedata channels and the hailing channels are mutually exclusive (i.e.,every data channel is different from every hailing channel). In oneembodiment, a subset of the data channels and the hailing channels maybe the same, while other data channels and other hailing channels may bedifferent. And in one embodiment, the set of data channels and hailingchannels may be the same. In one embodiment, the channel spacing may begreater or less than the 100 kHz spacing discussed above.

A non-limiting, exemplary set of 50 hailing channels (from hailingchannel 0 to hailing channel 49) includes the following frequencies:

Ch. Freq. Ch. Freq. Ch. Freq. Ch. Freq. 0  926.8 MHz 1 922.96 MHz 2925.48 MHz 3 922.72 MHz 4   922 MHz 5 925.96 MHz 6 922.84 MHz 7 922.48MHz 8 923.32 MHz 9   925 MHz 10  923.2 MHz 11 924.52 MHz 12 925.12 MHz13  922.6 MHz 14 923.68 MHz 15 925.36 MHz 16 924.16 MHz 17 927.76 MHz 18927.88 MHz 19  927.4 MHz 20 924.76 MHz 21 924.28 MHz 22 926.92 MHz 23926.44 MHz 24 927.16 MHz 25 922.63 MHz 26 924.04 MHz 27 923.92 MHz 28923.56 MHz 29 923.08 MHz 30 922.24 MHz 31 927.28 MHz 32  926.2 MHz 33926.08 MHz 34  923.8 MHz 35 924.88 MHz 36 925.24 MHz 37 925.84 MHz 38923.44 MHz 39 927.52 MHz 40 922.12 MHz 41 926.56 MHz 42 924.64 MHz 43927.64 MHz 44  924.4 MHz 45 927.04 MHz 46 926.68 MHz 47 925.72 MHz 48926.32 MHz 49  925.6 MHz

In one embodiment, these hailing channels may be grouped into hailingchannel groups. For example, hailing channel group 0 may include hailingchannels 0 and 1 (908.15 MHz and 919.8 MHz in the above example), whilehailing channel group 1 may include hailing channels 2 and 3 (922.65 MHzand 902.65 MHz in the above example), continuing through hailing channelgroup 24. More generally, hailing channel group “n” may include hailingchannel “x” and hailing channel “x+1” where “x” represents a hailingchannel. In other embodiments, hailing channel groups may include adifferent number or combination of hailing channels.

A non-limiting, exemplary set of 50 data channels (beginning with datachannel 0 and continuing through data channel 49) includes the followingfrequencies:

Ch. Freq. Ch. Freq. Ch. Freq. Ch. Freq. 0 922.94 MHz 1  922.1 MHz 2923.78 MHz 3 922.46 MHz 4  926.9 MHz 5 927.26 MHz 6 922.82 MHz 7  923.3MHz 8 927.86 MHz 9  927.5 MHz 10  923.9 MHz 11 926.42 MHz 12 925.46 MHz13 927.38 MHz 14  926.3 MHz 15  925.7 MHz 16  925.1 MHz 17 926.18 MHz 18925.94 MHz 19 924.02 MHz 20 927.98 MHz 21 926.66 MHz 22 924.98 MHz 23927.62 MHz 24 924.74 MHz 25 925.22 MHz 26 925.34 MHz 27 924.62 MHz 28 924.5 MHz 29 926.54 MHz 30 924.14 MHz 31 923.66 MHz 32 925.58 MHz 33922.22 MHz 34 924.26 MHz 35 927.02 MHz 36 922.34 MHz 37 926.06 MHz 38926.78 MHz 39 923.42 MHz 40 927.74 MHz 41 924.86 MHz 42 924.38 MHz 43 922.7 MHz 44 922.58 MHz 45 925.82 MHz 46 923.54 MHz 47 927.14 MHz 48923.18 MHz 49 923.06 MHz

In one embodiment, these data channels may be grouped into data channelgroups. For example, data channel group 0 may include data channels 0and 1 (922 MHz and 904.5 MHz in the above example), while data channelgroup 1 may include data channels 2 and 3 (908 MHz and 925 MHz in theabove example), continuing through data channel group 24. Moregenerally, data channel group “p” may include data channel “y” and datachannel “y+1” where “y” represents a data channel. In other embodiments,data channel groups may include a different number or combination ofdata channels. In one embodiment, the data channels are not grouped.

In one embodiment, the hailing channel frequency set and the datachannel frequency set are unique to each device/node within the system.However, in one embodiment, the hailing channel frequency set and thedata channel frequency set may be the same or contain a portion of thesame frequency channels. Each device/node has a “nodeID” whichidentifies it within the network. A device wishing to send acommunication to a target device utilizes the target device's “nodeID”to identify the correct hailing channel frequency set and data channelfrequency set to use for that particular target device. The “nodeID” isan identifier, such as an alphabetic and/or numeric string, associatedwith and unique to a device.

FIG. 1 is an exemplary block diagram of the network topology of an AMImesh network, including utility provider 102 and AMI devices includingRF devices 110, 112, 114, 116. The RF devices discussed herein may alsobe referred to as “AMI devices,” “nodes,” or “devices.” Theconfiguration of AMI devices shown in FIG. 1 is merely oneconfiguration, and additional devices or alternative configurations maybe used. As such, the configuration of AMI devices shown in FIG. 1should not be seen as limiting but instead as merely exemplary. Thedashed lines of FIG. 1 represent wireless communication links betweenthe devices 110,112,114,116 and the utility provider 102, which may beactive during some periods of time and which may not be active duringother periods of time, as will become clear throughout the presentdisclosure.

As seen in FIG. 2, an RF circuit 200 may be included in any of the AMIdevices shown in FIG. 1, such as RF devices 110, 112, 114, 116. RFcircuit 200 within each RF device 110, 112, 114, 116 enables the devicesto communicate wirelessly with one another. Battery 205 powers atransceiver integrated circuit (IC) 210, a microprocessor 220, an RFpower amplifier 230, an RF low noise amplifier 240, and flash memory250. Crystal oscillators 215,225 are connected to transceiver IC 210 andmicroprocessor 220, respectively. The circuit 200 includes atransmit/receive switch 260 and antenna 270. Although flash memory 250is specified, any type of memory such as RAM, ROM, flash memory, etc.may be used with RF circuit 200, as understood by those having skill inthe art.

A data line connects antenna 270 to the transmit/receive switch 260. RFreceived data from antenna 270 is fed into RF low noise amplifier 240and then to transceiver IC 210. Transceiver IC 210 is connected tomicroprocessor 220 and to RF power amplifier 230. If RF transmissiondata is to be sent to antenna 270 and, thereby, to another remotelylocated communicator (for example, from RF device 110 to RF device 112of FIG. 1), it is transmitted to the RF power amplifier 230 where it isamplified and transmitted to transmit/receive switch 260 and on toantenna 270 for communication. In one implementation, meter data iswirelessly received and transmitted to and from a host and remotelylocated meter nodes connected to water meters.

Microprocessor 220 and transceiver IC 210 include both a two-way dataand a two-way control line. Microprocessor 220 includes a control lineto each of RF low noise amplifier 240 and transmit/receive switch 260.Microprocessor 220 is also connected to flash memory 250 by both atwo-way data line and by a battery status line, the battery lineincluded so that flash memory 250 may notify microprocessor 220 of itspower and battery status. Finally, microprocessor 220 is connected to adevice circuit 280. In one embodiment, device circuit 280 may include autility meter such as a water meter or an electricity meter. In oneembodiment, device circuit 280 may be a reporting and testing device,such as a water or gas leak detection device. These examples are notmeant to be limiting, and those of skill in the art will recognize thatalternative device circuits may be used in conjunction with the presentdisclosure. Note, other supporting circuitry and memory may not be shownin the figures of this disclosure but would be understood by those ofreasonable skill in the art.

RF circuit 200 may be configured on various radio topologies in variousembodiments, including point-to-point, point-to-multipoint, meshnetworking, and Star, among others. RF circuit 200 may be configured tocommunicate in multiple topologies or in one of multiple topologies.

RF devices such as those shown in FIG. 1 (devices 110, 112, 114, 116)and FIG. 2 may be in one of 3 states at any given time: SLEEP state usedto conserve battery life; SLAVE state used for responding to andreceiving data from a MASTER state device; and MASTER state used toinitiate communications with (i.e., “hail”) and to send data to a SLAVEstate device. A device initiating a communication is referred to hereinas the “master” or the “MASTER state device” and the device which is thetarget of the communication is the “target,” the “target device,” the“slave” or the “SLAVE state device.”

Sleep State

FIG. 3 is an exemplary flow diagram 300 of the SLEEP state of an RFdevice. Some AMI devices (e.g., RF devices 110, 112, 114, 116) arebattery powered. To maximize the life of the device batteries, thesedevices are normally in SLEEP state until it is time to transmit and/orto receive data messages. In this state, the AMI device, such as RFdevice 112, is in a “deep sleep” (i.e., very low power mode) and wakesup once every 0.75 seconds to listen for a hailing signal from a devicein MASTER state, for example, such as RF device 110 in FIG. 1. Duringthis short listening period, 1-2 milliseconds, for example, the devicelistens to its currently-defined hailing channel group consisting of two(2) hailing channels from the hailing channel frequency set. While a twohailing channel configuration is disclosed, single channel or othermulti-channel configurations may be used. If radio frequency power(e.g., a hailing signal) is detected on one of the hailing channels, theAMI device then tries to decode a hailing message on that channel, thehailing message containing a pointer to a particular data channel (thedata channel pointer). Once a device in SLEEP state has received ahailing message, the device then synchronizes with the transmittingdevice (i.e. the master) through any remaining channels of the twohailing channel set and then switches to the proper data channel, asindicated by the data channel pointer. Once on the proper data channel,the SLAVE state device waits for a data channel message from the master.Once the data channel message is received, the target device respondswith an acknowledgement message (ACK message) on the same channel andthen switches to the next data channel in the data channel frequencyset.

Still referring to the exemplary method of FIG. 3, a device such as RFdevice 112 of FIG. 1, which may be battery powered, enters SLEEP stateupon the termination of a prior data transmission at block 302. At block304, the device waits for a period of time known as the “sleepTime”before proceeding to block 306. At block 306, the AMI device listens fora hailing signal on a first hailing channel. Selection of hailingchannels is discussed above in the Pseudorandom Frequency Sets section,and an exemplary hailing channel frequency set and data channelfrequency set are shown. In one embodiment, the present disclosure maydetermine whether to enter SLAVE state (enter data mode) at block 500after listening on each hailing channel individually.

After listening on the first hailing channel, the AMI device thenlistens for a hailing signal on a second hailing channel at block 310(and any subsequent hailing channels if more hailing channels arepresent in the hailing channel group). If a hailing signal is detected(block 312) on the second hailing channel, the AMI device enters SLAVEstate for receiving data at block 500, as discussed below. If no hailingsignal is detected (block 312) on the second hailing channel, the AMIdevice then prepares to enter the deep sleep during which the AMI devicedoes not attempt to detect a hailing signal. To enter this deep sleep,the AMI device increments its wait time counter (block 314), thendetermines whether the total hailing time (e.g. sleepTime×n) is lessthan the total channelPeriod (block 316). The total channelPeriod is theamount of time the AMI device utilizes one hailing channel or group ofhailing channels before moving on to the next. If so, the AMI deviceenters the deep sleep and waits a duration of sleepTime beforeattempting to detect a hailing signal again. In one embodiment, thesleepTime may be approximately 750 milliseconds; however this time maybe adjusted depending on the number of hailing channels used during eachcycle. If the total wait time is greater than or equal to apredetermined total channelPeriod (block 316), the AMI device incrementsits hailing channel counters at block 318 such that the first hailingchannel becomes a third hailing channel and the second hailing channelbecomes the fourth hailing channel, the hailing channels beingdetermined from the hailing channel frequency set. In one embodiment,the channelPeriod may be approximately 18 seconds, which would result inevery hailing channel—for example, 50 hailing channels when usedtwo-at-a-time—being used over a 7.5 minute period. As persons havingordinary skill in the art recognize, a different channelPeriod and/or adifferent sleepTime may be advantageous.

In another embodiment, an AMI device such as RF device 112 of FIG. 1 maybe a line powered device that does not need to conserve power and cantherefore utilize a more aggressive channel monitoring procedure than abattery powered RF device. Line powered AMI devices may continuouslysample all 50 of its hailing channels without waiting a sleepTimebetween sampling. In this configuration, the sampling method is similarto the battery operated device method shown in FIG. 3, but sampling iscontinuous. That is, both methods sample the RF power in each channel.When RF power is detected in one of the hailing channels, the unit willthen try to decode a hailing message on that channel. When a hailingmessage is successfully decoded, it contains a data channel pointer to aparticular data channel. Once an AMI device receives a hailing message,the AMI device then synchronizes with the transmitter through anyremaining channels of the two channel set. The AMI device then switchesto the indicated data channel and waits for a data channel message fromthe master. Once the data channel message is received, the AMI deviceresponds with an ACK message on the same channel. The device thenswitches to the next data channel in the data channel frequency set tocontinue receiving the data message.

Slave State

In an exemplary method of receiving data messages 400 shown in FIG. 4, atarget device in SLAVE state such as RF device 112 of FIG. 1 is awakeafter having synchronized successfully through being hailed by a devicein MASTER state, discussed below, such as RF device 110 of FIG. 1. Oncethe target AMI device transitions from SLEEP state to SLAVE state (block402), the target AMI device waits for data transmission from the AMIdevice in MASTER state (block 404). The target AMI device determineswhich data channel selected from its data channel frequency set to usebased on the data channel pointer sent during the hailing procedure bythe MASTER state device. When data messages are received (block 406),the SLAVE state device responds to the MASTER state device bytransmitting an ACK message on the same data channel (block 408) back tothe MASTER state device. After sending the ACK message (block 408), theSLAVE state device switches to the next data channel in the data channelfrequency set (block 410) to wait for more data from the MASTER statedevice (block 412). If the MASTER state device has additional data totransmit, it sends the additional data on the next data channel. Thecycle continues until the MASTER state device has no more data totransmit. Once the last data message has been received and the final ACKmessage has been sent (or the device times out waiting for a message),the SLAVE state device will return to the SLEEP state (block 414).

Master State

In an exemplary method of transmitting data messages 500 shown in FIG.5, an AMI device in MASTER state such as RF device 110 of FIG. 1 is inone of two modes of operation: hailing mode for hailing andsynchronizing with an AMI device in SLEEP state using the hailingchannel frequency set; and data mode for transmitting data to andreceiving data from a device in SLAVE state using the data channelfrequency set. As indicated above, the AMI device in MASTER state useshailing mode to synchronize communications with and to connect to theAMI device that is in SLEEP state. First, beginning at block 502, thedevice exits SLEEP state and enters MASTER state for synchronizing withand sending data to another AMI device. In hailing mode (block 504), theMASTER state device sends a hailing message on the hailing channel(s)currently being sampled (as determined from the hailing channelfrequency set, system timer, and nodeID) by the device that is in SLEEPstate (block 506). Each hailing message contains a data channel pointerindicating which sequentially next data channel to use from the datachannel frequency set. If the hailing attempt is unsuccessful (block508), the MASTER device attempts additional hailing attempts afterwaiting a delay period (block 507) and retries the hailing procedure asdiscussed herein with particular reference to FIG. 9.

After successfully hailing the SLEEP state device (which then entersSLAVE state) and synchronizing communications, the MASTER state devicewill switch to the indicated data channel (block 510) and send a datamessage to the target device (block 512). If an ACK message is received(block 514) from the SLAVE state device, the MASTER state deviceswitches to the next data channel (as determined from the predefinedpseudorandom data channel frequency set) and continues sending data. Ifno ACK message is received (block 514) by the MASTER state device fromthe SLAVE state device, the MASTER state device switches to the next setof hailing channels and retries hailing and synchronization (block 504).In one embodiment, the total occupancy of any channel will be less than400 milliseconds within a 20 second period.

Once in data mode, the MASTER state device sends data in each of itsdata channels, switching channels only after receiving an ACK messagefrom the SLAVE state device, until all data has been sent. The datalength is variable depending on the amount of data to be transmitted.Each new data transmission begins on the sequentially next channel inthe MASTER state device's normal data hopping sequence as determinedfrom the data channel frequency set. In one embodiment, data channelusage is therefore spread across the data channel frequency setsubstantially equally to minimize the channel occupancy and assure that,on average, all channels in the data channel frequency set are usedsubstantially equally. In one embodiment, it may be desirable to usecertain data channels for longer periods or more often than other datachannels, such as to use data channels with a lower interference levelmore frequently than data channels with a higher frequency. At the endof the data transmission, both the MASTER state device and the SLAVEstate device return to SLEEP state. In one embodiment, scheduledcommunications (uploads from a MASTER state device to other devices)have a minimum random delay time added to the start of hailing of, forexample, at least 7.5 minutes. The random delay time ensures that, onaverage, all of the hailing channels are used equally by beginning thehailing process at different channels on a random basis.

System Description

The following describes the operating mode and settings for radiofrequency enabled nodes/devices.

Battery Life Considerations—

For battery operated RF devices, the biggest drain on the projectedbattery life is caused by the periodic “sniffing” of hailing channelsand the time spent transmitting. To achieve the best system performance,the sniffing rate on the receiving side should be balanced with theamount of hailing that needs to be sent from the transmitting side. Withthis balancing in mind, it has been determined that an optimum systemperformance for minimum battery drain (and thus maximum battery life) isa “sniffing” rate of approximately 800 milliseconds and a hailingtransmission of approximately 800 milliseconds (this equates to two hailmessages). However, due to the way the timers within the processoroperate, it may be necessary in some embodiments to reduce the sniffrate to, for example, approximately 750 milliseconds. As persons ofordinary skill in the art recognize, other sniffing rates and/or hailingtransmission times may be required depending on hardware specificationsand environmental factors.

In theory, these rates produce a near 100% success rate for hailingbetween devices. However, in practice environmental interference anddevice hardware reduce the actual success rate somewhat. It isimpossible to predict the average success rate because the environmentalinterference is completely unpredictable, as those skilled in the artappreciate. In some embodiments, a first hail hit rate of better than90% is likely.

FIG. 6 shows a graph 600 of power usage in an exemplary system of thepresent disclosure. “WoR PWR” represents the average rate of currentdraw in milliamps (“ma”). In one embodiment, the current draw occurs inperiodic pulses. For example, in one embodiment, a pulse may be 10milliamps for 10 milliseconds, and that pulse may repeat at a rate ofonce every 1000 milliseconds. The current draw between pulses may beless, for example 1 milliamp, during the 990 millisecond period betweenpulses. In this example, the average current draw is calculated usingthe following equation:

$\frac{\left( {10\mspace{14mu} {ma}*10\mspace{14mu} {ms}} \right) + \left( {1\mspace{14mu} {ma}*990\mspace{14mu} {ms}} \right)}{1000\mspace{14mu} {ms}} = {1.09\mspace{14mu} {ma}}$

FIG. 6 shows the plot of the average power for the WoR, the averagepower used in the hailing process, and the sum of these two values. Inthis way, the optimum number of hailing channels required to minimizethe average current draw on the battery may be determined. As the numberof hailing channels increases, the number of channels sniffed increases,but these channels are “sniffed” less frequently. “Hail PWR” representsthe average milliamps used for one hailing attempt in a day. The powerincreases with the total number of channels used in the hailing attempt.TotalPWR is the sum of Hail PWR and WoR PWR.

System Clock—

In order for “hailing” to occur properly, the MASTER state device andthe SLEEP state device must know which hailing channels to use at anygiven time. As discussed, each device has a hailing channel frequencyset. When a MASTER state device needs to send data to another device,which is in SLEEP state, the MASTER state device selects a channel fromthe SLEEP state device's hailing channel frequency set based on a systemtime and the SLEEP state device's nodeID, which is known to the MASTERstate device. That is, the MASTER state device and the SLEEP statedevice share a common system time, and the hailing channel to be used isselected from the hailing channel frequency dependent upon the systemtime.

In one embodiment, the system clock (“sysClock”) may be the only timereference kept by a device. By itself, the sysClock time has no meaningrelative to wall clock time. Rather, it merely tracks the elapse of realtime. In one embodiment, the sysClock may be in synchronization withwall clock time. The sysClock is a counter maintained by the firmwarewithin each device and is based on a periodic signal generated by eachdevices' hardware (such as by oscillator 225 and microprocessor 220 inFIG. 2). The firmware is responsible for configuring the hardware ofeach device such that a periodic interrupt (the “sysTimer interrupt”) isgenerated, for example, every approximately 750 milliseconds in oneembodiment. The “sysTimer interrupt may vary in frequency, as recognizedby those having skill in the art.

The sysTimer interrupt is derived from a crystal and hardware basedprescaler. These crystals typically have an accuracy of 20 ppm˜100 ppm.However, to ensure reliability within the system, an accuracy of 5 ppmor less is preferred. For this reason, a more precise value for thesysTimer interrupt may be required, although other precision levels maybe advantageous depending on the particular embodiment as recognized bythose of ordinary skill in the art. The value of the sysClock isincremented each time the sysTimer interrupt is processed. The value ofthe increase is determined by the firmware calibration proceduresdescribed below in the Firmware Calibration section.

In one embodiment, a device may contain a hardware real-time clock (RTC)chip. The RTC chip is not used as the wall clock reference time, butinstead, the RTC chip serves as a backup to the sysClock. Since thesysClock is much more accurate than the RTC, the RTC is set from thesysClock reference at the start of each upload. If a unit with an RTCchip is reset, for example following a power failure, it then determinesif the value contained in the RTC is valid and if so, the device loadsits sysClock counter from the RTC chip. If the RTC chip time is notvalid, then the sysClock will restart from 0.

Firmware Calibration—

In the time-based hailing of the present disclosure, the time driftbetween any two nodes should be, for example, approximately one secondper day or less. Many of the current commercially available productshave hardware not capable of supplying the required accuracy; therefore,a firmware calibration may be needed to alter the sysClock to achievethe desired accuracy and reliability. As those skilled in the artrecognize, the firmware calibration process should be executedperiodically to maintain the required accuracy.

A firmware calibration may be performed by comparing a high-accuracyclock reference generated from a radio frequency temperature-compensatedcrystal oscillator (TCXO) (for example, crystal oscillator 225 in FIG.2) with an accuracy of approximately 2 ppm (known as “ref_clock”) to thesysClock used to produce the sysTimer. In order to satisfy the accuracyrequirement, the sampling time and ref_clock may include:

${\frac{\frac{1}{{ref\_ clock}{\_ freq}}}{sample\_ time}*10^{6}} \leq {5({ppm})}$

where “ref_clock_freq” is the frequency of the reference clock derivedfrom the TCXO and “sample_time” is based on a number of sysTimerinterrupts.

Since the sysTimer interrupt is derived from the hardware crystaloscillator within the RF device (for example, crystal oscillator 225 inFIG. 2), its accuracy is the same as the crystal oscillator, which maybe, for example, approximately 100 ppm. In order to achieve theheightened precision required by the hailing process of the presentdisclosure, the actual value of the sysTimer interrupt may bedetermined. This is done by counting the number of ref_clock cyclesduring a sample time. A more accurate value of the sample time can thenbe calculated by multiplying the number of ref_clock cycles counted bythe period of the ref_clock (“ref_clock_count”):

${ActualSampleTime} = {{ref\_ clock}{\_ count}*\frac{1}{{ref\_ clock}{\_ freq}}}$

The true value of the interrupt can then be calculated by dividing theactual sample time by the number of interrupts used to generate thesample time:

${TrueInterruptValue} = \frac{ActualSampleTime}{NumberOfInterrupts}$

The value of the sysClock is then increased by the true interrupt valueeach time the interrupt is processed.

UTC Time—

Coordinated Universal Time (“UTC”) is calculated by adding autcTimeOffSet variable to the current sysClock time. The utcTimeOffSetvariable is stored, in one embodiment, in EPROM. As those skilled in theart recognize, other types of memory may be used. The value of theutcTimeOffSet is the difference between the local sysClock and the UTCtime received from a device in MASTER state during the hailing procedureas described herein. The utcTimeOffSet variable is updated periodically(e.g., at least once per day at the start of an upload attempt). UTCTime is defined as timeSync plus the utcTimeOffSet:

UTCtime=sysClock+utcTimeOffset

Timesync—

Timesync data contains a four-byte value that represents the source UTCtime (e.g., which RF device sent the UTC time). When a device receivestimesync data and resolves to use that information, it calculates a newutcTimeOffSet value as follows:

utcTimeOffset=(timeSyncData)−(sysClock)

Timesync information is always included in both a ping message (sent bythe MASTER state device) and a pong ACK message (sent by the SLAVE statedevice). In this way, both the MASTER state device and the SLAVE statedevice exchange timesync information as part of the hailing procedure.Each device only uses timesync information if it is sourced from one ofits “parent” nodes. Otherwise, the timesync information is discarded bythe “child” node unless the child node has no parents. In this case, thereceived timesync information is used by the child node regardless ofthe source of the timesync. Ultimately, the parent node is the masterclock and hailing timer source for all of its children. The parent musthave a stable timing reference that allows only minimal timer shifts topropagate through the network via its children. This can be achievedusing the same firmware calibration procedure described in the FirmwareCalibration section above. In one embodiment, the parent may utilize thestandard simple network time protocol (SNTP) to retrieve timeinformation from a remote data source such as the Internet. The parentmay update its time once per day, for example, or on some other periodicschedule.

FIG. 7 is a flow diagram showing the process 700 relating to a devicereceiving a timesync from another device. In one embodiment, the device(such as RF device 112 shown in FIG. 1) polls to determine whether itreceives a timesync (block 702). If not, the device continues to poll(block 702). If the device does detect a received timesync at block 702,the device determines whether it has a parent device (such as RF device110 shown in FIG. 1) (block 704). If the device does have a parent asdetermined at block 704—or multiple parents in one embodiment—the devicethen determines whether the timesync was sent by its parent (or one ofits parents) (block 706). If so, the device updates its utcTimeOffSet toequal the received timesync from its parent (block 708). Upon updatingits utcTimeOffSet, the device returns to polling for a new timesync(block 702). However, if the device has a parent (or multiple parents),and the timesync was not sent by one of the device's parents, the devicerejects the timesync and continues polling for a new timesync (block702). If the device has no parents, however, it accepts any receivedtimesync and updates its utcTimeOffSet to equal the received timesync(block 708). Upon updating its utcTimeOffSet, the device returns topolling for a new timesync (block 702).

In one embodiment, the device may not poll and instead timesyncinformation is received via the hailing signal, which may include thetimesync information.

Restoring UTC Time after Reset—

For AMI devices that contain a hardware UTC device, after any reset(such as through regular maintenance or through prolonged power outage)the AMI device verifies that the UTC time is valid first by checking tosee if the oscillator stop flag (“OSF”) (such as for oscillator 225shown in FIG. 2) is set in the device. If the OSF is set, then the UTCtime in the device is not valid. If this is the case, the device usesdata from the RTC chip to recreate the sysClock and verify that theresulting UTC time calculation returns a date after the stored firmwarerelease date. The present disclosure utilizes the firmware release date,which represents the date that the firmware load was released toproduction from the manufacturer, to verify the system time. Forexample, when the system time is lost, such as because of a prolongedpower outage, the value of the RTC chip may show a time many years inthe past. The firmware release date provides a reference date forcomparing to the system clock to ensure that the new system time isreasonable. If the date is before the firmware release date then thetime is invalid. If a device cannot restore a valid sysClock from an UTCdevice, then it may request a valid timesync from a parent device.

System Hailing Timer—

The system hailing timer is a virtual timer within each RF device thatoperates continuously. In one embodiment, as shown in FIG. 8, the systemhailing timer period is a 450 second timer; however, other timer valuesmay be desirable as those of reasonable skill in the art can appreciate.In one embodiment, the value of the system hailing timer is calculatedby adding the utcTimeOffSet variable to the current sysClock value plusan offset calculated from the node ID and then performing a modulooperation on that result equal to the timer period (for example, 450),as follows:

systemHailingTimer=(sysClock+utcTimeOffSet+nodeIDoffset)%timerPeriod

For a device in SLEEP state, when calculating which hailing channels tomonitor, the local nodeID is used to calculate the offset. For a devicein MASTER state, when calculating which hailing channels to use totransmit a hailing signal, the target SLEEP state device's nodeID isused to calculate the offset. The resulting value will point to one ofthe hailing channels selected from the hailing channel frequency set.This timer is common between nodes and controls the selection of bothtransmitting and monitoring/receiving hailing channels.

FIG. 8 shows a diagram of an exemplary hailing sequence using a 450second system hailing timer period. The hailing timer period is brokeninto 25 hailing channel time periods of 18 seconds each. In this way,the 50 hailing channels from the hailing channel frequency set arepaired into 25 channel pairs. Each hailing channel pair is assigned to ahailing channel time (e.g., hailing channel time 0, hailing channel time1, . . . , hailing channel time 24). Each hailing channel time is 18seconds long in one embodiment, meaning that a SLEEP state device willattempt to detect a hailing signal on only these two channelsperiodically for a period of 18 seconds. The hailing method is furtherdescribed herein and shown, in particular, in exemplary flow diagram 300of the SLEEP state in FIG. 3.

Super Sniffing Mode—

In one embodiment, whenever a device receives a new timesync, regardlessof the source, and the resulting time shift is more than a predefinedthreshold, for example, 7 seconds, the device enters “super sniffingmode.” During super sniffing mode, the device attempts to detect ahailing signal on all 50 of its hailing channels sequentially with nowaiting period between attempts, rather than using the hailing methoddisclosed for SLEEP state hailing. This embodiment enables the device toapply a new timesync from its parent more quickly and prevents thedevice from being disconnected from its own children. This embodimentfurther enables faster testing and debugging of the network when a validtimesync from a parent is unavailable, for example, during maintenance.When the source of the time shift is a parent of the device, the devicestays in super sniffing mode for an extended period, for example, 24hours. In other cases, the device stays in super sniffing mode for asignificantly shorter period, for example, 20 minutes. Upon completionof super sniffing mode, the device returns to SLEEP state and normalhailing presumes.

Parent Node Operation

In one embodiment, a parent node such as device 110 in FIG. 1 may beresponsible for hundreds or even thousands of child nodes such asdevices 112, 114, 116, etc. of FIG. 1. As used herein, “parent” and“child” nodes should not be confused with “MASTER state” and “SLAVEstate” devices. MASTER state and SLAVE state are merely states/modes foreach device. “Parent” and “child” nodes have a predefined relationshipbased on hierarchy (i.e., a parent node such as device 110 in FIG. 1 isfurther up the hierarchy in relation to the utility provider 102 than achild node such as devices 112, 114, 116 in FIG. 1). Although thepresent disclosure describes a single parent to multiple childrelationship, it should be understood that multiple parents may existwithin the same network. Further, a child may have multiple parents. Invarious embodiments, a single parent may be paired with a single child.As an example, child nodes may represent individual customers' utilitymeters while a parent node may represent a primary data collectiondevice responsible for collecting date from and sending data to eachchild device. This configuration requires that the parent device'ssystem time be highly accurate.

Calibration—

In one embodiment, a parent is assumed to have a more accurate time thanits children. The parent therefore becomes a “time master” for theentire cell, the cell including the parent and each of its children, inone embodiment. To maintain clock accuracy, the parent device runs thesame calibration algorithm as described above on its radio as describedherein.

Real Time—

In one embodiment, a device includes two processors; however, as thoseof reasonable skill in the art recognize, different numbers ofprocessors may be advantageous depending upon the desiredimplementation. In a two-processor embodiment, a first processor (e.g.,a “rabbit” processor) facilitates Internet communications, and a secondprocessor (e.g., the radio processor) facilitates network communicationsbetween parent and child devices, such as those described herein betweenMASTER state, SLAVE state, and SLEEP state devices.

Upon initialization, the rabbit processor sets the radio processor'stime if the rabbit processor knows the time. The rabbit processorfurther sets the radio processor's time once per day. In one embodiment,the rabbit processor may include a double battery back-up clock whichmaintains time across extended power failures. In one embodiment,accuracy of the rabbit processor may be limited to a 32 kHz crystal,which is approximately 20 ppm˜100 ppm.

The rabbit processor may use the standard SNTP protocol, as discussedabove, to retrieve time from the Internet. In another embodiment, otherprotocols may be used as known in the art. In an embodiment using theSNTP protocol, the Rabbit updates its time, for example once per hour orafter a system reset. Time updates may be immediately applied ordelayed. The rabbit system time is limited in one embodiment to shiftingno more than, for example, 9 seconds after each new Internet time updateunless its internal time is invalid, in which case it will apply thefull time shift regardless of the time shifting limit. In this way, badtime shifting settings that would disrupt the network, for example alarge shift in time such as minutes or hours, may be prevented.

In one embodiment, a parent may automatically push the time information(timesync) to its children when hailing, causing the children to updatetheir time information (timesync). If the parent does not know the time,it will clear the pong timestamp in its hailing messages to notify thechildren not to update their clocks. In one embodiment, certain parents(for example, parents closer to the utility provider in the networkhierarchy) may have their automatic time-syncing disabled and may onlybe time-synced when required, not automatically.

Hailing Channel Groups—

In one embodiment, a hailing channel group is, for example, a set of twocontiguous channels from the hailing channel set. In a configurationusing 50 hailing channels, there are 25 channel groups that arenon-overlapping and contiguous. For example, channels 0 & 1 representgroup 0, channels 2 & 3 are group 2 and so on until the last twochannels 48 & 49 are used to make group 24. Referring now to FIG. 8,“hailing channel time 0” represents the hailing channel time for group 0(i.e., channels 0 and 1), “hailing channel time 1” represents thehailing channel time for group 1 (i.e., channels 2 and 3), and “hailingchannel time 24” represents the hailing channel time for group 24 (i.e.,channels 48 and 49). As persons of reasonable skill in the art willappreciate, different channel groupings (such as, for example, 3channels per group) and/or different numbers of total channels may beadvantageous in other embodiments.

As shown in FIG. 8, based on the hailing timer value, a device in SLEEPSTATE monitors each hailing channel group (monitored for an incominghailing signal from a MASTER state device) for 18 seconds, for example.Each RF device determines a starting channel group based on its nodeId.

Whenever the hailing timer wraps back to channel group 0 (i.e., channels0 and 1), the device begins monitoring its starting channel group. Eachchannel group is monitored for 18 seconds, for example. Upon completionof each monitoring period, the device begins monitoring the nextsequential channel group (shown as “channel time” in FIG. 8). After thedevice monitors channel group 24, the device monitors channel group 0.In an embodiment having 25 channels groupings of 2 channels per group,with each channel group being monitored for 18 seconds, it takes 450seconds (7.5 minutes) to cycle through all 25 channel groups. Inembodiments with other numbers of channels and/or channel groupings orwith differing amounts of monitoring time, the total time to cyclethrough the entire channel grouping will vary accordingly. It istherefore possible to determine which channel group a device ismonitoring based on the value of the hailing timer (sysTimer).

Hailing Attempts—

When a device (i.e., a device in MASTER state or a master device)determines it is time to send information (i.e., a data message) toanother device in the network (i.e., a target device, which is in SLEEPstate), the master device first “hails” the desired device using thehailing method described herein. In this way, the devices achievechannel hopping alignment or become synchronized. In this configuration,the device initiating the communication is the “master” and the devicewhich is the target of the communication is the “slave.”

In order to maintain approximately equal use of hailing channels withinthe hailing channel frequency set, any periodic communication betweennodes, such as a daily upload, may have a random start delay applied tothe first hailing attempt. In one embodiment, the random start delay mayrange from 0 to 13 minutes. Asynchronous messages such as on-demandutility meter readings do not require additional random delay andhailing can begin immediately.

To hail a slave device, the master device first determines which hailingchannel group the slave is monitoring. It does this by using the slave'snodeId to determine what the slave's starting channel group is. Next themaster calculates which, for example, 18 second window the systemhailing timer represents and adds that value to the slave device'sstarting channel group. Once the master has determined which hailingchannel the slave is using, it sends its first hail attempt, whichconsists of one hailing message sent on each of the hailing channels inthe hailing channel group. The transmission of a single hailing messageoccupies, for example, approximately 395 milliseconds in atwo-channel-per-group configuration, so the pair of hailing messageswill occupy, for example, approximately 790 milliseconds. This hailingattempt spans the entire sniffing window of the slave (i.e.approximately 750 milliseconds, as discussed in the Slave State andSleep State sections above). In this way, the slave device will alwayshave performed a channel sniff during a hail attempt. This helps ensurethat, with the exception of local interference and/or background noise,it is reasonably certain that the slave device will detect the masterdevice's hail attempt. In one embodiment, the first hailing attemptshould not start in the first 2 seconds or last 2 seconds of the hailingwindow. This allows for a 1 to 2 second difference between devices,which can be caused by drift or delays in passing time sync information.

The hailing message contains a pointer to the master device's nextchannel selected from the data channel frequency set. After sending thehailing message, the master device switches to the selected data channeland sends a synchronize (ping) message. It then waits for a responseacknowledgement (ACK or pong) message from the target device. If noresponse is received, the master retries the hail request. In oneembodiment, this retry occurs during the next 18 second hailing windowusing the next hailing channel group. If the slave again fails torespond, the master device may continue attempting to hail the targetdevice. If after a predetermined number of retries (for example, 3retries) the master still has not received an ACK from the targetdevice, it performs a “back-off and retry” procedure described in theHailing Back-Off and Retry section below with reference to FIG. 9. Ifthe master device receives an ACK message response from the targetdevice, it first processes any timesync information, then switches tothe next data channel and begins normal data communications. In oneembodiment, a battery powered master device returns to the SLEEP statebetween each hailing attempt to conserve battery life. In an embodimentwith a continuously powered master device, the master may not return toSLEEP state between each hailing attempt because no battery lifeconservation is necessary.

Hailing Back-Off and Retry—

If, after completing a set of hailing retries as discussed, the masterdevice has still failed to initialize communications with the targetdevice, the master device backs-off for a random time (for example,between 2 and 13 minutes). After waiting this random time, the masterdevice then performs another full hailing attempt and hailing retrycycle. If the master device is still unable to contact the targetdevice, the master device may perform additional back-off and retryattempts. In one embodiment, a total number of retry attempts may be set(for example, 5 retry attempts). In this case, upon reaching the totalof retry attempts, the master device will cease trying to contact thattarget device for a period of time (for example, 24 hours). In oneembodiment, the master device may send an alert message with theparticular target device's nodeID to the utility provider that it wasunable to establish communications with that particular target device.

In an embodiment wherein an RF device has multiple parents, the aboveprocess of hailing, hailing retries and back-off, and retries isattempted for each configured parent until either all configured parentshave been tried or successful contact is established with a parent.

FIG. 9 depicts a flow diagram of an exemplary embodiment of the hailingand retry process used by an RF device in MASTER state as describedherein. At block 902, the device determines whether it is time to uploaddata and sets its retry counter to zero. If it is not time to upload,the device continues to determine whether it is time to upload at block902. If it is time to upload, the device determines whether it is timeto perform hailing and sets the hailing counter (HailCnt) to zero (block904). If not, the device continues polling whether it is time to performhailing (block 904). However, if it is time to hail, the device jumps tothe first hailing channel selected from the hailing channel frequencyset at block 906. Next, at block 908, the device sends a hailing messageto the target device and proceeds to the next hailing channel (block910). At block 912, the devices sends another hailing message to thetarget device and hops to the appropriate data channel selected from thehailing channel frequency set (block 914).

Once on the data channel, the master device sends to the target device a“ping” message (block 916). If the target device successfully receivesthe ping message and returns an ACK (“pong”) message, which is receivedby the master device, the master device processes any necessary timesyncinformation, as described herein, (block 928) and then proceeds tocomplete the data transmission (block 930).

If the target device does not respond to the master device's “ping”message by sending an ACK (“pong”) message (block 918), the masterdevice attempts a re-hailing of the target device if the HailCnt is lessthan a maximum hailing attempt value, for example, 3 hailing attempts(block 920). If the HailCnt is less than the maximum hailing attemptvalue, the master device waits until the next hailing time (block 922)and begins the hailing process again on the next hailing channel (block906). However, if the HailCnt is equal to or greater than the maximumhailing attempt value, the master device will restart the entire uploadattempt beginning at block 902 for a predetermined number of retryattempts (for example, 3 retry attempts) (block 924). Once the totalnumber of retry attempts has been exceeded, the master device will waita random delay period (for example, 2 minutes to 13 minutes as discussedabove) (block 926) and begin the process again.

Hunting—

When a master and slave are out of timesync (that is, when the sysTimerwithin the master is not in sync with the sysTimer of the slave), thenthe master does not know what hailing channel or hailing channel group aslave may be sniffing at any given time. When this occurs, in order tomake a connection to the slave, the master may send a hailing message onall of the hailing channels, starting with the master's next hailingchannel. Due to the additional time required to send 50 hails, the hailmessage contains an indication so that the slave will be aware of theadditional hailing. After a hailing message has been sent on all hailingchannels in the hailing channel frequency set, the master hops to theindicated data channel and sends a sync request (ping) message to theslave. The slave responds with an ACK message (pong) on the same datachannel then hops to the next data channel in the data channel frequencyset to begin data communications.

This process of “hunting” for a slave device uses a significant amountof battery power, so limiting the use of hunting as much as possible isdesired. Hunting is only used for system requests and only after a setof normal hailing attempts has failed. It is also used when a particulardevice declares an extended outage as described below. Hunting causesone of the most significant network delays as well as causing asubstantial drain on the battery life.

Extended Communications Failure Recovery—

In one embodiment, each RF device maintains in memory a setting for howmany days must pass without communicating with any parent after whichthe device enters into an extended outage mode. For example, this timeperiod may be 10 days, although in other embodiments that duration maybe shorter or longer, as persons having ordinary skill in the art canappreciate. If a device has failed to hail successfully its configuredparent(s) for more than the extended outage period, it then assumes thatit has lost time synchronization with its parent(s), causing the deviceto enter the extended outage mode.

When in the extended outage mode, the device no longer attempts anyautomated messaging (i.e., uploads, alerts, etc.) with its parent(s).The device does continue to hunt for its primary parent once, forexample, every 5 days until the device successfully hails its parent.Once this occurs, the device exits extended outage mode. Battery-powereddevices that experience frequent extended communications failures alsoexperience reduced battery life because the hunting process requiressignificant battery power.

On Demand/Relayed Communications—

In one embodiment, a device, such as device 110 of FIG. 1 may receive anasynchronous data message (such as, for example, an installation relatedmessage or an on-demand meter read request) from a source farther up thehierarchy, for example, from utility provider 102, with instructions torelay that data message to another device, for example, device 112. Whena device attempts to forward an asynchronous relayed message, the devicefirst performs a hailing attempt with the target device and, ifunsuccessful, a hailing retry. If the target device responds to any ofthe hailing attempts, then the device forwards the relayed data messageto the target device. Once the data message is sent to the target deviceand the target device sends the ACK message back to the sending device,the sending device returns to the SLEEP state.

If the target device fails to respond, the sending device initiates a“hunting” attempt as described in the Hunting section above. In oneembodiment, the sending device initiates the hunting attemptimmediately. However, in another embodiment, the target device may waita predetermined or random amount of time before initiating the huntingattempt. If the hunting attempt fails, the sending device returns anindication of failure (NACK) to the source of the relayed message. If adevice is successful in contacting the target device, it forwards therelay message to the target device and waits for a reply (i.e., an ACKmessage). If a device that receives a relayed message (i.e., the targetdevice) is unable to process the request (such as because of a lowbattery), the target device returns a negative acknowledgement (“NACK”)message to the sending device indicating that an error has occurred and,in one embodiment, specifying the cause of the error. Otherwise, uponthe successful receipt of the relayed message, the target deviceresponds with the ACK message to the sending device and then forwardsthe message as necessary or process the message itself if it is thefinal intended target of the message.

During installation, most devices are not in time synchronization withone another, and therefore a hunting procedure for each newly installeddevice is required. Once a device has been contacted through the huntingprocedure described herein, it receives a hailing timer synchronizationmessage. Since, during installation, a device likely does not have anassigned parent yet, the timesync information is applied immediately andno additional hunting is required, saving battery life.

In one embodiment, the following per synchronization delays may beexpected:

Sync Time Number of (seconds) Hailing Attempts Min Max 1 1 4 1 + 1 retry11 25 1 + 2 retry 29 43 1 + 3 retry 49 61 1 + 3 retry + hunt 70 81

Parents Assigned by Field Radio—

In one embodiment, a parent device may be configured by an installationtechnician using a field radio. Since the device does not have a parentinitially assigned, the device accepts and immediately applies atimesync message received from the field radio operated by a technician.

Once receiving the timesync message from the field radio, the installeddevice attempts to contact its assigned parents, which may occur atpreconfigured upload times. If a parent has already been installed witha valid timesync, then the installed device begins receiving and usingtimesync information from its parent. If the parent is not available,the installed device performs periodic attempts to contact the parent.In one embodiment, if the number of days without contacting theinstalled device's parent reaches the configured extended outagethreshold, it begins a hunting procedure, described above in the Huntingsection, for its parent. Once communication with the parent has beenestablish, the parent will send timesync information to the child (theinstalled device) and normal network communications may then follow.

Network Outage Recovery Examples

FIG. 10 depicts a block diagram of a simple network infrastructurestemming from utility provider 1010 having a plurality of nodes 1001,1002, 1003, 1004, 1005, 1006, 1007, 1008. In this configuration, node1001 is a parent to node 1002; node 1002 is a parent to nodes 1003,1004; node 1003 is a parent to nodes 1005, 1006; and node 1004 is aparent to nodes 1007, 1008. The following are examples of some commonactions which may be performed within a network implementing the systemsand methods disclosed herein. As these are merely examples, they are notintended to be limiting. As persons of ordinary skill in the art willrecognize, other configurations may be possible or otherwiseadvantageous.

Node Swap—

In this example, node 1002 has stopped functioning and needed to bereplaced by a technician. When new node 1002 is placed into the network,its sysClock may not be in synchronization with the rest of the network.Depending on the amount of time that node 1002 was out of service, it isalso possible that the sysClocks of node 1002's children (nodes 1003,1004) as well as their respective children are no longer insynchronization with one another or with node 1001.

At the time when the new node 1002 is installed, it is not configuredwith node 1001 as its parent. Therefore, new node 1002 accepts timesyncinformation from any node. In this way, the device can be configuredfrom utility provider 1010 through the network or locally using a fieldradio. Either way it is configured, it is assumed that the timesync thatthe child device receives will be in synchronization with the networktime and, therefore, in synchronization with the sysClock of node 1001.

Nodes 1003 and 1004 may also need to update their parents also. In oneembodiment, this could be accomplished through a system message fromutility provider 1010 routed through nodes 1001 and 1002. In anotherembodiment, this may be accomplished through a message routed from afield radio through new node 1002. In either case, new node 1002 mayperform a hunting procedure as described above to connect to itschildren (nodes 1003 and 1004). In the case of node 1003, as soon as thehunt is successful, node 1003 receives timesync information from node1002, its parent. If the time shift is substantial, node 1003 entersinto super sniffing mode, as disclosed herein, which enables itschildren (nodes 1005 and 1006) to continue to connect to it. The samewill happen when node 1004 is hunted. Nodes 1005, 1006, 1007, and 1008will all get the new timesync on their next upload. This procedureensures that, upon completion of replacing node 1002 with new node 1002,all nodes should be able to upload virtually immediately.

Extended Outage Recovery—

In this example, node 1002 in the sample network shown in FIG. 10 isunable to communicate with any other node in the system for a period oftime. At the time that node 1002 recovers, its sysClock may not be insynchronization with the rest of the network. It is also possible that,if the node was out for an extended period of time (for example, morethan 10 days), nodes 1003 and 1004 and their respective children arealso out of timesync relative to node 1001. When this occurs, if node1002 has a hunting delay set to zero, then node 1002 requires assistancefrom an external device in order to rejoin the network. However, if node1002 has a hunting interval set (not to zero), then node 1002 attempts ahunting procedure to connect with its primary parent (in this example,node 1001) at it programmed interval. If the fault is clear, the huntingprocedure results in node 1002 receiving a valid timesync from itsprimary parent (node 1001). This terminates the extended outage andclears the corresponding error. Node 1002 then begins its normaluploading procedure.

If node 1002 was out for less than the expected period of time (forexample, 10 days or less), the hunting procedure is not necessarybecause the sysClock of nodes 1001 and 1002 are aligned sufficiently toenable a normal hailing procedure between the nodes to establishconnection when necessary. As soon as node 1002 re-connects to node1001, node 1002 receives an updated timesync from node 1001. If the newtimesync causes a significant time shift (for example, a time shiftgreater than 7 seconds), node 1002 enters super sniffing mode (i.e.,node 1002 begins sampling all of its hailing channels for an extendedperiod of time).

This same process occurs between node 2 and its children (nodes 1003 and1004). Since the children of nodes 1003 and 1004 (nodes 1005,1006, 1007,and 1008 respectively) did not experience a loss of connection to theirparents, these nodes are not in the extended outage mode and use thenormal hailing procedure to upload data messages, as disclosed herein.If the time shift passed from node 1002 to its child nodes is small (forexample, 1-2 seconds), then the child nodes' sysClocks may be alignedclosely enough for the nodes to connect without issue (i.e., the nodesmay continue to use the normal hailing procedures). However, if the timeshift is large (for example, more than 10 seconds), then node 1002samples all of its hailing channels. Again, the child nodes have notrouble connecting and uploading their data. All nodes in the networkshould be able to upload their data as soon as the outage clears.

Dynamic Squelching

In one embodiment, it may be desirable to increase receiver sensitivityto reduce the effects of unwanted background noise on an RF device.However, doing so causes an increased drain on battery life. The presentdisclosure further provides a controlled method to balance battery lifeagainst receiver sensitivity. Generally, this is done by adjusting a“noise floor” to control the “false break rate” as described below. Whenthe false break rate is increased, the sensitivity increases but thebattery life decreases. Conversely, when the false break rate isdecreased, the sensitivity decreases but the battery life increases. Inthis way, the dynamic squelching disclosed herein aids in controllingthe number of false breaks on individual hailing channels in order tomaximize battery life while concurrently maintaining a high sensitivitylevel for each hailing channel.

Background noise may come from many sources such as cordless telephones,baby monitors, garage door opening devices, and any device operating onthe RF spectrum. Background noise varies depending on thefrequency/channel. The “noise floor” and the “break level” shouldtherefore be maintained for each channel separately. The method of thepresent disclosure further minimizes the number of false breaks due totransient interferers, which are noise sources present for a relativelyshort period of time. These transient interferers cause the measuredradio frequency power on a particular channel to increase above thenoise floor. An example of a transient interferer is a cordless phone,which may be in use for shorts durations of a few minutes to an hour orso at different and variable times. The break level described hereinquickly adjusts to eliminate this interferer from causing excessive“false breaks” on the RF devices of the present disclosure. As soon asthe interfering device has ceased causing interference on a particularchannel/frequency (e.g., the cordless telephone is no longer activebecause a call has ended), the method of the present disclosure returnsthe RF devices to the desired sensitivity level or noise floor.

As shown in FIG. 11 depicting the dynamic squelching method 1100,battery-operated radio frequency devices periodically wake up and listenon one or more hailing channels while in SLEEP state (block 1102). Whileattempting to detect a hailing signal on a selected hailing channel, themicroprocessor of the RF device requests a Receive Signal StrengthIndicator (RSSI) measurement from the RF device's chip set for theselected hailing channel (block 1104). The RSSI measurement is anindication of how much energy is present in the selected hailingchannel. One RSSI measurement is requested for each currently-monitoredhailing channel, which in a preferred embodiment may be two channelsselected from the hailing channel frequency set but may be a differentnumber of channels in other embodiments as recognized by those ofordinary skill in the art.

A device fully awakens (i.e., the device enters SLAVE state to listenfor a data communication from a MASTER state device) whenever thereported RSSI measurement exceeds the calculated break level (blocks1106 and 1108). A “false break” occurs when a device fully awakens(i.e., enters SLAVE state) (block 1110) due to the reported RSSImeasurement on a particular hailing channel, but there is no actualnetwork traffic (i.e., no data to receive) on that particular hailingchannel (i.e., no MASTER state device is attempting a data transmissionto or requesting information from the SLAVE state device) (block 1112).In this case, a “false break” occurs (block 1114). False breaks aremainly caused by background noise existing in the environment around theRF devices of the present disclosure. Following a false break, the RFdevice returns to SLEEP state. Excessive “false breaks” unnecessarilydecrease battery life of the RF device.

In one embodiment, if the RSSI measurement is lower than the break levelfor a certain number of times (or cycles through the hailing process),the noise floor calculation may be adjusted downwardly at block 1118.Continually monitoring the RSSI value allows the RF device to track andknow the long-term noise level on each channel in order to adjust thenoise floor accordingly, thereby maximizing sensitivity once a transientinterfere has subsided.

The RSSI measurements are used to calculate a noise floor level and abreak level for each hailing channel. The number of RSSI measurementsrequired for each RF device depends on the total number of hailingchannels used. In a preferred embodiment having 50 hailing channels inthe hailing channel frequency set, a total of 50 measurements arenecessary. In one embodiment, the noise floor level and the break levelare maintained in the RF device's memory for each hailing channel, evenwhen a channel is not actively being sampled.

The “noise floor” is based on the RSSI value measured for a particularchannel. The RF device's microprocessor slowly adjusts the noise floorsetting to achieve a programmable ratio of the number of times thesquelch is broken (i.e., the number of false breaks) to the total numberof times that the device checks to see if the RF device is being hailed.In this way, the long-term noise level in each channel may be known andtracked so that the “squelch” can be adjusted to substantially equal thenoise floor for maximum sensitivity once a transient noise has subsided.Since the number of hailing attempts performed each day is known basedon the system timing described herein, the ratio of false breaks tototal hailing attempts can also be converted to a number of false breaksper day. The purpose of the noise floor is to maintain maximumsensitivity to incoming RF signals while concurrently minimizing thebattery drain due to processing false wakes caused by background noiseinterference. In a preferred embodiment of the present disclosure, adefault noise floor ratio of approximately 800 false breaks per day isdesirable. However, in other embodiments, a different default noisefloor ratio may be desirable. If the noise floor is set too low, the RFdevice receives too many false breaks and wastes battery life byprocessing them unnecessarily. Conversely, if the noise floor is set toohigh, valid communications sent by a MASTER state device may be ignoredbecause an RF device may not have enough power to exceed the noise floorlevel, effectively reducing the communication range between devices.

During the hailing and dynamic squelching process (as shown in FIG. 11),the RSSI measurement is compared to a break level (i.e., a “squelchlevel”) for the associated hailing channel (block 1108). If the RSSImeasurement is above the break level, then the unit fully awakens andattempts to process an incoming data message from a device in MASTERstate (block 1130). If no data message is being sent, then the action isdeclared a false break (block 1114). The current break level for thatchannel is then set to the measured RSSI value (block 1116), and thenoise floor calculation is adjusted (1118). The device then enters SLEEPstate and resumes the hailing and dynamic squelching procedure disclosedherein (block 1120). This prevents short-term interferers that cause anRSSI measurement above the noise floor from causing multiple falsebreaks.

If a valid message is received (blocks 1112 and 1130), no changes aremade to the break level or noise floor. If the measured RSSI value isequal to or below the break level but above the noise floor value forthe channel, no changes are made to the break level. Only when themeasured RSSI is below noise floor level for the channel is the breaklevel adjusted down to equal the noise floor level for a channel.

A fast-reacting dynamic squelch, in one embodiment, may aid inpreventing a transient noise source from causing multiple false wakes(i.e., prevent an RF device from continually waking up to listen on ahailing channel that does not contain valid information for the RFdevice). This fast-reacting dynamic squelch is set to the RSSI levelmeasured by the RF devices microprocessor, as described above. In oneembodiment, and as shown in FIG. 11, the fast-reacting dynamic squelchlevel remains substantially constant until a higher RSSI level isdetected, thereby causing a new false break, in which case thefast-moving dynamic squelch is set to the new RSSI level. In oneembodiment, the fast-reacting dynamic squelch remains substantiallyconstant until the RSSI level is measured to be substantially equal toor below the noise floor. In this case, the fast-reacting dynamicsquelch level is set to the noise floor level.

In one embodiment, a long-term interferer may be present near an RFdevice of the present disclosure. A long-term interferer, unlike atransient interferer, may be an RF device that utilizes one or more ofthe channels used in the hailing process of the present disclosure, withsuch use continuing for extended periods of time. Unlike transientinterferers, which may only produce background noise or interference forshort and/or sporadic periods, the long-term interferers tend to producebackground noise or interference for extended, and in some casesindefinite, periods of time. When a long-term interferer is present nearan RF device of the present application, the long-term interferer cangreatly affect battery life by continually causing false breaks to occurduring the RF device's hailing process. That is, every time the RFdevice listens for a hailing signal on the channel in which thelong-term interferer is present, the RF device would wake-up to listenfor a data message where no such data message is present (i.e., a falsebreak). To increase battery life of the RF device, it is necessary toeliminate the interference caused by the long-term interfere.

When the RF device detects a long-term interferer through prolonged highRSSI values in a particular hailing channel, the RF device adjusts thenoise floor so that the RF device will not detect any signals on theparticular hailing channel. In one embodiment, the RF device may includelogic adapted to communicate to the other devices in its network not touse that particular channel any longer or to use a substitute hailingchannel instead.

One should note that conditional language, such as, among others, “can,”“could,” “might,” or “may,” unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements and/or steps. Thus, suchconditional language is not generally intended to imply that features,elements and/or steps are in any way required for one or more particularembodiments or that one or more particular embodiments necessarilyinclude logic for deciding, with or without user input or prompting,whether these features, elements and/or steps are included or are to beperformed in any particular embodiment.

It should be emphasized that the above-described embodiments are merelypossible examples of implementations, merely set forth for a clearunderstanding of the principles of the present disclosure. Any processdescriptions or blocks in flow diagrams should be understood asrepresenting modules, segments, or portions of code which include one ormore executable instructions for implementing specific logical functionsor steps in the process, and alternate implementations are included inwhich functions may not be included or executed at all, may be executedout of order from that shown or discussed, including substantiallyconcurrently or in reverse order, depending on the functionalityinvolved, as would be understood by those reasonably skilled in the artof the present disclosure. Many variations and modifications may be madeto the above-described embodiment(s) without departing substantiallyfrom the spirit and principles of the present disclosure. Further, thescope of the present disclosure is intended to cover any and allcombinations and sub-combinations of all elements, features, and aspectsdiscussed above. All such modifications and variations are intended tobe included herein within the scope of the present disclosure, and allpossible claims to individual aspects or combinations of elements orsteps are intended to be supported by the present disclosure.

1. A wireless communication network comprising: a plurality of nodes,each node being configured in wireless communication with at least oneother node, and at least one of the plurality of nodes being configuredin wireless communication with a utility provider; wherein the utilityprovider and the plurality of nodes are arranged in a hierarchy with theutility provider being highest in the hierarchy, the plurality of nodesbeing arranged such that two nodes in direct wireless communication witheach other include a parent node and a child node, the parent node beinghigher in the hierarchy with respect to the child node; and wherein,when a first node of the plurality of nodes is determined to beout-of-service, the plurality of nodes are configured to perform arecovery procedure to restore the first node to operation, the recoveryprocedure including transmitting a time synchronization signal from asecond node to the first node, the second node being in direct wirelesscommunication with the first node.
 2. The wireless communication networkof claim 1, wherein the recovery procedure includes: sending a hailingmessage on a plurality of hailing channels from a second node to thefirst node, the second node being in direct wireless communication withthe first node.
 3. The wireless communication network of claim 2,wherein the recovery procedure further includes sending an acknowledgemessage from the first node to the second node.
 4. The wirelesscommunication network of claim 2, wherein second node sends the hailingmessage on 50 hailing channels.
 5. The wireless communication network ofclaim 1, wherein the recovery procedure includes supplying a timesynchronization signal from a field radio to the first node.
 6. Thewireless communication network of claim 1, wherein the first node isdetermined to be out-of-service when it fails to communicate with othernodes for at least a first predetermined length of time.
 7. The wirelesscommunication network of claim 6, wherein the first predetermined lengthof time is ten days.
 8. The wireless communication network of claim 6,wherein, if the first node resumes communicating with the other nodesbefore the first predetermined length of time, the first node receives atime synchronization signal from one of the other nodes to enable thefirst node to continue operation.
 9. The wireless communication networkof claim 8, wherein the first node is determined to be out-of-service ifthe time synchronization signal received from the one of the other nodescauses a time shift greater than a second predetermined length of time.10. The wireless communication network of claim 1, wherein the firstnode is determined to be out-of-service when the first node is a newnode replacing a non-functioning node.
 11. The wireless communicationnetwork of claim 1, wherein at least one of the plurality of nodes isassociated with at least one meter for measuring utility usage for theutility provider.
 12. A radio frequency (RF) device of a plurality of RFdevices configured within a wireless communication network, the RFdevice comprising: an oscillator operatively configured to maintain areference clock signal; an antenna operatively configured to transmit RFcommunication signals directly to and receive RF communication signalsdirectly from at least one other RF device of the plurality of RFdevices configured within the wireless communication network; and amicroprocessor operatively connected to the oscillator and the antenna,wherein the microprocessor is operatively configured to determine when afirst RF device of the at least one other RF device is out-of-service,wherein the microprocessor is further configured to perform a recoveryprocedure to restore the first RF device to operation, and wherein therecovery procedure includes transmitting a hailing message via theantenna on a plurality of hailing channels to the first RF device. 13.The RF device of claim 12, wherein the recovery procedure furtherincludes transmitting the reference clock signal via the antenna on afirst data channel to the first RF device and receiving an acknowledgemessage from the first RF device on the first data channel.
 14. The RFdevice of claim 12, wherein the microprocessor determines the first RFdevice to be out-of-service when it fails to communicate for at least apredetermined length of time.
 15. The RF device of claim 12, wherein themicroprocessor determines the first RF device to be out-of-service whenthe first RF device is newly installed within the wireless communicationnetwork to replace a non-functioning RF device.
 16. A method ofwirelessly communicating within a communication network, the methodcomprising the steps of: communicating directly and wirelessly with atleast one node of a plurality of nodes in the communication network;determining when a first node of the at least one node isout-of-service; and performing a recovery procedure to restore the firstnode to operation, wherein the recovery procedure includes transmittinga time synchronization signal to the first node.
 17. The method of claim16, wherein the recovery procedure includes the steps of transmitting ahailing message on a plurality of hailing channels to the first node.18. The method of claim 16, wherein the step of determining when thefirst node is out-of-service includes the step of determining when thefirst node fails to communicate for at least a predetermined length oftime.
 19. The method of claim 18, wherein, if the first node resumescommunicating before the predetermined length of time, the step ofdetermining when the first node is out-of-service further includes thesteps of: transmitting a time synchronization signal to the first nodeto enable the first node to continue operation; and determining if thetransmitted time synchronization signal causes a significant time shiftin the first node.
 20. The method of claim 16, wherein the step ofdetermining when the first node is out-of-service includes the step ofdetermining when the first node is a new node configured to replace anon-functioning node.